Process of preparing a ferrous alloytype powder for powder metallurgy and of preparing high strength articles therefrom



asstaz Patented Dec. 4, 1962 lice PRDCESS F PREPARKBIG A FERRUUS ALLSY-TYPE PGWDER FQR POWDER METALLURGY AND 0F PREPARENG EHGH STRENGTH ARTI-CLES TEEREFRGM William A. Reed, West Richfield, and Sheridan R.

(Iroeirs, (Ileveiand Heights, Ohio, assignors to Republie SteeiCorporation, Cleveland, Ghio, a corporation of New Jersey No Drawing.Fit-ed Mar. 24, 1960, Ser. No. 17,251

11 Claims. (Ci. 75-211) The present invention relates to a process ofpreparing a ferrous alloy-type powder for powder metallurgy, and moreparticularly for preparing such a powder capable of being formed intoarticles by more or less conventional powder metallurgy processes andwherein those articles have unusually high tensile strengths, both assintered" and after subsequent heat treatment. The powder produced bythe process of the present invention is also characterized by lowshrinkage, in that the reduction in any linear dimension of a partformed therefrom between the time of the completion of the compressionof the powder in a mold and the completion of formation of the partincluding sintering and possibly also subsequent treatrnents is reducedto a tolerable minimum.

The present invention also includes the process of preparing highstrength articles from powder prepared in accordance with thisinvention.

As such, the present invention constitutes an improvement on the subjectmatter of United States Patent No. 2,799,570, granted July 16, 1957.

This prior patent disclosed and claimed the co-reduction of iron-oxide,nickel in the form of oxide and/or chloride, and molybdenum or manganeseor possibly both in metallic form or in a hydrogen-reducible form. Whilethe powder of this earlier invention exhibited desirable strengthcharacteristics when made into parts, the shrinkage thereof as abovedefined was considered by some commercial users of this product to beexcessive in that it Was about 0.8% to about 1% in many instances. It isdesired that this shrinkage be reduced to about 0.4% or less. This isaccomplished in accordance with the present invention.

A major distinction in substance between the disclosure of the earlierpatent aforesaid and the present process lies in the fact that the iron,which is the principal ingredient of the powder, is not co-reduced alongwith the other reducible materials present, but rather is supplied tothe present process in a reduced state and in the 0 form of additiveiron, which is a particular and pecu liar type of iron powder as will beset forth in greater detail hereinafter. As such, the present processmay sometimes be termed an after treatment as it is carried on followingthe substantial completion of the reduction of the iron to its metallicform.

Also, the present process, as well as the process of the earlier patentaforesaid, may not be strictly stated as being for the preparation of atrue alloy powder, as it seems wholly probable that the metals presentare not all in complete solid solution in each other. There is, however,as far as is known, some alloying of the metals resent, although this isprobably something less than 100% complete as has been evidenced bymicrophotographic examination of many samples of finished parts or testpieces made from the powder of the present invention. It is believed,therefore, more accurate to term it an alloy-type powder than a truealloy powder.

As generally referred to hereinabove, the process of the presentinvention produces a powder which not only has relatively high strengthand low shrinkage characteristics when made into articles and tested onan as sintered basis, but also, such articles may be further treated,for example, by heat treatment steps, including tempering withsubsequent drawing of the temper to a predetermined desired extent.Also, the powder of the present invention may be further densified, ascontrasted with conventional pressing and sintering processes used inpowder metallurgy, by pressing first at a pressure which may berelatively low or relatively high (it is more or less immaterial which),then annealing at a selected temperature, followed by a re-pressingoperation at a pressure which is relatively high, preferably about80,000 p.s.i. to 100,000 p.s.i. or more, concluding with the usualsintering operation. This produces a very dense body, usually having adensity substantially over 7 gms./cc., and also having relatively hightensile strength. Any desired special heat treatment steps may beperformed on the re-pressed blank or body prepared as aforesaid. Allthese subsequent treatment steps are to be included as parts of thepresent invention.

The process of the present invention is particularly characterized bythe materials used therein, which must be of the kind and present in theproportions specifically set out herein. These materials may besummarized as: manganese in a metallic form and present in an amountfrom about 0.1% to about 1%; molybdenum in the form of at least onehydrogen-reducible compound thereof and present in the amount of about0.1% to about 1%; nic..el in the form of either metallic nickel or ahydrogen-reducible compound therecf and present in an amount from about/2% to about 2 /z%; and the balance additive iron containing not overabout 1% oxygen as oxide impurities. These materials in suitableparticle size as hereinafter set out in detail are dry mixed, thenintroduced into a reducing zone and there exposed to a reducing gascontaining a substantial proportion of hydrogen, while being kept at atemperature in the range of about 1200 F. to about 1600 F. for a timesufficient to efiect the reduction of substantially all thehydrogenreducible material present, usually about 30 to 40 minutes. Thepowder is then cooled under non-oxidizing conditions, so as to preventspontaneous combustion thereof. This powder so prepared may betransmitted to its point of use as such.

At the point of fabrication of articles from the powder, it is normallymixed with from about /2% up to about 1% carbon, usually in the form ofgraphite, and with a mold lubricant, such as Zinc stearate, then pressedat a desired pressure and sintered. The present invention'may in someinstances also include the pressing and/ or sintering steps or anysubsequent heat treatment steps. These steps will be described to acertain extent as they are also used in making practical tests of theessential characteristics and potentialities of the powder, which isvaluable by reason of its peculiar properties when pressed and sinteredand made into final articles. The powder of the present invention may beused to form articles which will have tensile strength in the order ofmagnitude of 80,000 p.s.i. on an as sintered basis, and up to about200,000 p.s.i. or more on a pressed, annealed, re-pressed, sintered andheat-treated basis.

The details of the requirements or" the present process will now bediscussed individually.

The first ingredient of the starting material to be discussed is theiron. This iron must be reasonably pure in order that it he usable inthe present process with the maximum desired physical characteristics inthe final product. Thus the impurities, principally acidinsoluble incharacter, and consisting of one or more of the oxides of aluminum,silicon, titanium, chromium, calcium and magnesium, should be present ina total amount of not more than about 1% based upon the weight of theiron present. This limitation serves praccally to preclude many powderediron materials which are produced directly by reducing iron ore withoutgoing through a melting process. A preferred source of the iron for useas the iron in the present process is mill scale, which is a by-productor" iron and steel fabrication and which is usually available insufiicient quantities to make the powder required in accordance with thepresent process. It is not meant to preclude positively all ironprepared by direct reduction. from iron ore as long as some process orprocesses are carried on with respect to this ore or with the reducediron product thereof which will keep the oxide impurities within thelimits hereinabove set out, i.e. not over about 1% total (calculated asweight of oxygen and substantially equivalent to hydrogen loss).

Another reason which is present for using mill scale as a source of theiron in this case is that most mill scale contains a proportion ofmanganese, so that when the mill scale is reduced to metallic form,much, if not all, of the desired manganese content for the finishedproduct will be found to be contained therein and a minimum amount ofmanganese need be added.

The next requirement as to the iron is that it be additive iron. This isa coined term which has been used to describe iron made in a particularway and usually substantially in accordance with the teachings of thepatent to Crowley, No. 2,744,002, granted May 1, 1956. The term cameinto existence in order to describe generally the character of the ironso produced.

If one visualizes particles of iron oxide reduced b exposing them in areducing zone to hydrogen, the hydrogen Will react with the oxygen ofthe iron oxide, abstracting this oxygen therefrom and leaving a porousor spongytype particle structure without refilling the spaces formerlyoccupied by the oxygen of the iron oxide. This type iron formed byabstracting oxygen from iron oxide may be termed subtractive iron, dueto the fact that the oxygen is subtracted therefrom and the resulting Uparticles are porous in structure although outwardly resembling in shapeand external dimensions the shape and corresponding dimensions of theOriginal iron oxide particles. Such particles are characteristic of sometypes of iron powder presently being sold commercially.

In accordance with the teachings of the Crowley patent aforesaid. wheniron cxide is reduced in a reducing zone at a sufficiently hightemperature so that the reaction proceeds, by the use of a gascontaining not only hydrogen, but also gaseous HCI, it is apresentrtheory, which has considerable support, that the iron oxidereacts with the HCl to form ferrous chloride; that this ferrous chlorideexists at least instantaneously in vapor form (due to the relativelyhigh vapor pressure of ferrous chloride at the temperatures used in thereducing zone); and. that this ferrous chloride reacts with hydrogenin'the vapor phase, so that metallic iron is deposited as a result ofthis reaction onto some minute nuclear particles of iron, with theresult that these particles are progressively built up as the reactionproceeds in a manner similar to the growth of a crystal. The particlesof this type iron, which have been termed additive iron,

are not spongy or porous, but rather are solid and sometimes more orless dendritic in character. Due to the apparent building up or growthof the iron particles as lowest tensile strength.

aforesaid, the iron powder so produced is termed additive iron.

It has been found that the subtractive iron formed as aforesaid may betermed relatively hard, while the additive iron may be termed relativelysoft. By these terms, usually indicative of the degree of hardness orsoftness, is meant in this art the characteristic or ability of an ironpowder to be compressed, so that for a given pressure a softer iron willmake a more dense pressed body than a harder iron. Also to make apressed body of a given bulk. density, a softer iron powder will requireless compacting pressure than a harder iron powder. It is desired inaccordance with the present invention that the iron be quite soft andthat the density of the pressed body thereof, sometimes termed the greendensity will be quite high, usually the higher the better.

The term additive iron, therefore, as used throughout the presentapplication and the appended claims, is intended to import ironparticles or powder formed in a way so that the particles per se willnot be porous to any substantial extent, but will be of the character ofthose made in accordance with the teaching of the Crowley patentaforesaid. It has been found that the use of a substantial amount, ifnot all, of additive iron in the present process is essential, so thatthe raw material for the present process is properly described toinclude iron, predominantly in the form of additive iron.

In general, a softer powder which can be pressed with a given pressureinto a more dense green pressed body will give a final article having atensile strength which is relatively high, the green pressed densitybeing generally indicative of the final tensile strength.

Again, some detailed investigations have indicated that the amount ofHCl present in the gases during the reduction of the iron oxide affectsthe shrinkage characteristics of the final product. For example, whenthe iron oxide is reduced in a relatively high l-lCl concen tration,i.e. about 3% or more, the finished powder will have the best (lowest)shrinkage characteristic, but the When the powder is prepared from ironoxide using a relatively low HCl concentration in the reducing gases,the shrinkage characteristics of the final product will be lessdesirable, although the strength will be somewhat greater. Here again,there is a balance, which must be effected, between shrinkage andtensile strength, recognizing that the attainment o these twocharacteristics respectively seems to be varied in inverse relation bythe amount of I-iCl used during the reduction of the iron oxide. Ingeneral, it is found that medium values of HCl concentration are bestfor most purposes, thus obtaining the best average characteristics as toboth tensile strength and shrinkage.

The particle size of the iron ingredient of the starting material is notnarrowly critical. It has been found, for

example, that if less than about 30% of the iron powder used as startingmaterial is minus 325 mesh in size (standard Tyler mesh), the strengthof the final product tends to be reduced; while if'over 60% of this ironpowder is minus 325 mesh in size, the shrin e tends by reducing millscale (a preferred embodiment of the .present' process), much if not allof the manganese is present thereinalloyed with the iron, so that when emill scale isfully reduced, the manganese in the starting material ispresent probably alloyed with the iron and in metallic form. If it isnecessary to add manganese in order to obtain the desired: proportionsof this element, any form of metallic manganese may be used, which willinclude any or all the several known and/or available iron-manganesealloys and also will include metallic manganese. The principles applyingto iron as to particle size seem to apply more or less equally to themanganese, so that particle size i not a particularly criticalcharacteristic as to the manganese ingredient.

As stated hereinabove, the proportion desired for manganese is fromabout 0.1% to about 1%, these proportions being calculated as the Weightof metallic manganese based upon the weight of all the metals present(calculated as metals). In general, these limits are chosen on the basisthat as the proportion of the manganese is increased above about 1%, theshrinkage characteristic of the final product gets to be too high. Thus,in a test wherein 2% manganese was used, the shrinkage was about 0.9%.The lower limit of 0.1% was set due to the fact that as the amount ofmanganese is reduced below about this amount, the final product haspoorer characteristics as to its ability to be heat treated and thetensile strength of articles made therefrom tends to be reduced. Thepreferred amount of manganese is about /z%.

The next ingredient to be considered is molybdenum. This material issupplied in accordance with the present invention as ahydrogen-reducible molybdenum compound, e.g. molybdenum oxide and/ orchloride. Molybdenum should be present to the extent of about 0.1% toabout 1%, again calculated as metal and based on the total of all metalspresent. These limits are chosen for the following reason: as to themaximum limit, there seems to be no appreciable benefit to be gained byusing more than about 1% and as molybdenum and its compounds arerelatively expensive, there is an obvious economic limit at about thisvalue. The use of relatively high values of molybdenum up to about 1%are, however, usually desirable because as the amount of mol bdenumpresent is increased within the limits given, the

product exhibits lower shrinkage characteristics, which is desirable.The minimum limit of about 0.1% is chosen for about the same reason asthe minimum limit in the case of manganese, namely, that as the amountof molybdenum is decreased, the ability of parts made from the finalproduct powder to be heat treated is more or less correspondinglyreduced, so that molybdenum contributes to what may be termed the heattreatability of the final product or of articles made therefrom.

The particle size for the molybdenum and/ or the compounds thereof, asintroduced, has been found to be relatively unimportant. This isbelieved to be true for the reason that at the temperature to which themolybdenum is subjected during the sintering of articles formed from thepowder of this invention, it is the present theory that the molybdenumis effective more or less in a vapor phase; and that it probablysublimes to some extent. It is found, however, that this vaporizationaction, if the theory stated is correct, does not result in losing anysubstantial proportion of the molybdenum initially present. It isbelieved that at least 90% of the molybdenum originally present in thesolid starting material finds its way into the final product.

One satisfactory form of molybdenum for use in accordance with thepresent invention is molybdenum oxide (MoO )pigment grade, which isabout 95% 325 mesh in size. The preferred amount of molybdenum used inaccordance with the present invention is about Considering now thenickel ingredient, this metal is supplied in the form of metallic nickel(in fine powder form) or in the form of one or more of thehydrogenreducible compounds thereof, such compounds practically beingthose selected from the group consisting of the oxides and chlorides ofnickel.

In this connection, it may be mentioned that when the oxides of a metalare referred to herein, it is contemplated that any compound, such asthe carbonate, which would be reduced to and/ or through the oxide statewhen subjected to reduction, is to be considered generally as equivalentof the oxide, as such a compound when exposed to the temperature of thereducing zone in accordance with the present invention will be reducedfirst to the oxide and then to the metallic state. This applies to anymetallic oxides usable in accordance with the pres ent invention, suchas molybdenum and/or nickel oxides.

As stated generally hereinabove, nickel should be present to the extentof about /2% to about 2 /2%. Nickel is used generally to impart strengthcharacteristics to the final product. The minimum proportion of nickel,therefore, is that proportion below which the strength imparted therebyis too small to comply with the requirements for the product of thepresent invention. Thus, powders, having a content of nickel less thanabout /2% wilt have tensile strengths on an as sintered basis of under60,060 p.s.i., which is generally considered undesirable from the pointof view of the present invention. 0n the other hand, manganese can actto some extent to replace nickel in supplying the necessary tensilestrength, so that low values of nickel are permissible with relativelyhigh values of manganese. The maximum limit as to nickel of 2 /2% isdictated by the fact that as the amount of nickel present is increased,the shrinkage characteristics also increase and at this value for nickelcontent, the shrinkage reaches a. tolerable limit according to thepresent invention. Another thing which affects shrink age is theparticle size of the nickel. Thus as the particle size of the nickeloxide, for example, and subsequently that of the reduced nickel formedtherefrom, is decreased, the shrinkage is generally increased. Again,this limits the particle size in which the nickel may be introduced.

The preferred amount of nickel in accordance with the present inventionis about 2% and the particle size not over about 75% of minus 325 mesh.Particle size is, however, not be considered as narrowly critical inthis respect.

Once the solid starting material has been decided upon and theingredient materials made available in accordance with the principleshereinabove set out, these several materials, all of which are inrelatively fine powder form, are mixed or blended together while dry andby the use of any suitable means, severai of which are known in the art.Such means may, for example, take the form of a rotating drum, similarto a concrete mixer, and of appropriate size in view of the size of theoperation in question. he intimately mixed and blended materials arethen introduced into a reducing zone, which may comprise any suitabletype of gas-to-solid contact equipment, i ciuding, for example, rotatingdrums, shaft furnaces, tray furnaces or even fluidized bed equipment,all of which are known in the art and which per se form no part of thepresent invention.

While in this reducing zone the temperature of the solid materials ismaintained at a desired value or within a desired range of values, suchvalue or values being in the range of about 1290 F. to about 1600 F. andpreferably about 1509" F. The limits of this range are chosen on thebasis that if the temperature is too low, the desired reduction of thehydrogen-reducible compounds is either too slow or non-existent. in thisconnection it is believed, as a matter of theory, that the heatingaction in the presence of a reducing gas as hereinafter set forth,serves not only to effect the chemical reduction of thehydrogen-reducible material present, but also to soften the metallicconstituents by a sort of annealing action, which may possibly alsocontribute to the forming of some true alloys, although the extent ofthis alloying is usually indefinite and difficult if not impossible todetermine. In any event, the heating action serves to render the reducedpowder very soft as this term has been used herein'and as it is definedabove.

The upper limit of this temperature range is chosen on the basis that asthe temperature is progressively raised,

there is an increasing tendency for the powder to cake or sintertogether. if this trend were allowed to proceed too far, there would notresult a loose, flowable powder as is desired, but rather a sinteredporous cake, which is undesired, and which could be reduced to powderform only at the expense of considerable work and cost and possibly alsowould result in work-hardening of the resulting powder. For this reason,therefore, the upper temperature limit is placed at about 1600 F.

The reducing action must take place in the presence of a gas containingsubstantial proportions of hydrogen, even though this gas may also haveother and preferably incrt ingredients Such as nitrogen. Excellentresults are obtained, for example, using cracked ammonia gas. Othersources of hydrogen which may or may not be reasonably pure may also beused including, for example, gases obtained as a by-product of thereforming of oil in so-called platinum reformers." it is found, however,that increasing amounts of methane are somewhat detrimental to theprocess, in that they appear to cause increasing shrinkagecharacteristics in the final product.

The time for the reducing treatment should be such as to permit ofsubstantial completion of the reducing reaction and is, of course, afunction of the temperature, so that with relatively higher temperaturesWithin the range given, relatively shorter times are required andviceversa. With preferred temperatures and gas compositions, timeperiods in the order of magnitude of 30 to 40 minutes are usuallyadequate. it will be understood, of course, that there must besufficient hydrogen available to react stoichiometrically with all theoxygen and/or chlorine present in combination with thehydrogen-reducible metals, and preferably an excess of hydrogen overthis exact stoichiometric amount. In the event that the gas beingemployed in the reducing zone is relatively loW' in hydrogen, relativelylonger times are usually required.

It is noted that it is possible that the nickel and/or molybdenum may beintroduced in the form of chlorides. If this is done, the product of thereduction reaction will obviously be hydrogen-chloride gas. However, ithas been found in practice that the amount of hydrogenchloride gas whichmay reasonably be produced and/or present at any one time from thissource is so small as not substantially to ailcct the type of ironpresent. As noted hereinabove, if the iron is reduced or in some Wayformed in the presence of hydrogen chloride gas, there results additiveiron. To a certain extent at least, additive iron may be produced by anafter treatment of subtractive iron as hereinabove defined with a gasincluding a substantial amount of HCl. If the iron used in the presentprocess is sufficiently converted to an additive, it may be adequate forthe present process even though this conversion may not be arrived atexactly as in the Crowley patent aforesaid. On the other hand, if whatis supplied to the present process is a commercial form of subtractiveiron, of which there are several, the amount of hydrogen chloridepresent as the result of either or both the molybdenum and/or nickelbeing introduced in the forms of their chlorides would be whollyinsulficient to convert any substantial portion of the subtractive ironto additive iron. Thus the iron must be supplied in the desired form ofadditive iron.

Once the heat treatment and/or reduction has been substantiallycompleted in accordance with the principles herein outlined, the powderis complete except that it must be brought down to substantially roomtemperature under non-oxidizing conditions, as otherwise it wouldoxidize spontaneously if,'for example, itwere exposed to air at the hightemperature at which it is treated as aforesaid. This is usuallyaccomplished by cooling the material while in any suitable non-oxidizinggas, for example, the same gas used during the reduction.

it has been found that the powder made in accordance with the presentinvention is sufliciently soft, so that it will be relatively dense whengiven a compacting pressure within normal operating values, for example,when articles formed from the powder of the present invention arepressed at 60,000 p.s.i., the density of the resulting compacts isusually from about 6.30 to about 6.35 gms./cc.; while at 80,000 p.s.i.pressure, green densities of about 6.57 to about 6.65 gms./cc. areusual; while at pressure of 150,000 p.s.i. green densities of about 6.95to about 6.98 grns./cc. have been obtained.

In order to get relatively hightensile strengths in articles made frompowder in accordance with the present invention, it is usually desiredto use a certain amount of carbon in the final articles. This carbon,while possibly present to some very small extent in the powder preparedas aforesaid, is usually introduced for most part after the powder hasbeen completely prepared as hereinabove set forth and prior to thepressing of the powder into the desired shape. It is quite usual forexample, to mix the powder with carbon in the form of powdered graphite.The amount of carbon to be introduced may, in some instances, slightlyexceed the amount desired .in the final article for the reason thatduring the subsequent sin tering operation, some carbon may react with asmall amount of oxygen present in the powder in the form of one or moreof the oxides of one or more of the metals present to form one or theother of the gaseous oxides of the carbon, which are dissipated duringthe sintering. it will' be understod that practically all metalpowdenparticularly ferrous metal powder, has some oxide contents whichare usually represented by what'is known as hydrogen loss. This isdetermined practically by heating the powder with hydrogen at a certaintemperature and for a certain time, with the loss in weight resultingfrom this treatment representing the so-called hydrogen loss. The powdermetallurgy fabrication industry usually requires that the hydrogen lossshall not exceed 1 /2%, although good metal powder usually has avery'much lower hydrogen loss value than this.

Also in preparing the powder for actual fabrication; into articles,it'is quite usual to mix therewith about 1% of a mold lubricant, such aszinc stearate. The addition of carbon in this way and the addition of amold lubricant will be understood to have been accomplished in the sameway on each of the several test specimens described in the exampleswhich follow. In the case of zinc stearate, substantially the entirematerial is believed to be volatilized during the subsequent sinteringor heat treatment operation, so that the addition thereof serves merelyduring the pressing operation as a lubricant and does not otherwiseaiiect the properties of the finished product.

There is further provided as a part of the present invention a processfor obtaining very high densities and relatively high tensilestrengthseven without special heat treatment. To this end, the processcomprises pressing the powder prepared in accordance with this inventionas aforesaid first at any desired pressure, usually about 50,000 to100,000 p.s.i.; then anneal the pressed blank at temperatures, which maybe as low as l275 F. in some instances and as high as about 1500 F. toISO-0 F. in

. other cases, depending on the type of results to be desired.

Following this annealing operation, which takes place in a reducing gasatmosphere or in an endo-gas atmosphere (which is specially designed tomaintain the carbon content of the articles substantially constant), theannealed article is then re-pressed, usually at a relatively highpressure, for'example, from about 80,000 to about ent invention willhave tensile strengths in the order of magnitude of 80,000 to 100,000p.s.i., and sometimes more in the case of pressed, annealed andre-pressed blanks or articles.

In some instances it may be desired to subject the as sintered articlesto a heat-treating step, somewhat similar to conventional heat treatingpractices. Thus, it may be desired to bring such articles up topredetermined soaking temperatures, for example, from about 1500 toabout 1600" F. for to minutes, then quench the so-heated articles in aquenching oil, which is heated to about 100 to 150 F, then temper thearticles, for example, by holding them for 20 minutes at a temperatureof from 400 to 600 F. The heat treated articles thus produced will havevery high tensile strengths, sometimes as high as about 200,000 p.s.i.,as set forth hereinafter in examples which follow.

From the point of view of shrinkage, it is found that the dimensionalchange of articles made from powder produced in accordance with thepresent process is in many instances not over and sometimes less than0.4%, which value has been set up as a standard to be met by somecommercial users of metal powder. Articles subjected to subsequent heattreatment as aforesaid usually shrink somewhat more during this heattreatment, so that it is usual, for example, to expect a shrinkage ofabout 0.15% greater for a heat treated article than for an as sinteredarticle.

The present invention will be further illustrated by the followingexamples:

EXAMPLE I This example illustrates certain of the preferred embodimentsof the present invention. In accordance with one preferred embodiment,additive iron powder was used, which was made in accordance with theteachings of the Crowley patent aforesaid and produced by reducing millscale (which contained an amount of manganese approximately equivalentto /2% in the final product of the present invention). To this was addedsufficient molybdenum in the form of M00 pigment grade (about 95% 325mesh) to give /2% molybdenum in the final product, and nickel oxide inthe form of NiO (75% 325 mesh) in an amount sufficient to give about 2%nickel in final product. The several powdered materials were thoroughlymixed together, then heated at 1500 F. for about to minutes in a gasformed by cracking gaseous ammonia. The heated material was then cooledin the same gas to give the desired powder.

In testing this powder, it was mixed with 1% graphite and 1% zincstearate and pressed at 60,000 p.s.i. to form a green pressed blank,which had a density of 6.4 gms./cc. This blank was sintered in anendo-gas for 30 minutes, the endo gas serving to maintain the carboncontent of the materials substantially constant and at about 0.85%carbon, the sintering being carried on at a temperature in the range ofabout 2030 to 2060 F. The endo gas used in this test had a compositionas follows:

The pressed and sintered blank thus formed was then tested and was foundto have a tensile strength of 60,000 p.s.i. This blank was alsocarefully measured to determine the shrinkage, which was found to beabout 0.4%.

Other tests were conducted with powders of substantially the sameidentical composition, treated in substan tially the same way as aboveset out, except that the pressure was 80,000 p.s.i. In certain of thesetests the form in which the nickel was introduced was varied ashereinafter set forth. The results of the tests are shown in thefollowing table.

Table I Test Piece sintered in Test Piece sintered in Exo Gas Endo GasRun sintered Shrinlr- Tensile Sintcred Shrink- Tensile Density age,Strength, Density age, Strength, (gm/cc.) Percent p.s.i. (gm/cc.)Percent p.s.i.

6. 59 0. 30 78, 000 6. 59 0. 81, 000 6 60 0.50 82, 000 6.62 0.65 88, 0006. 63 0.30 84, 000 6. 66 0. 45 92,000 6. 54 0. 20 66, 000 6. 56 0. 3465, 000 6. 52 0. 31 74, 000 6. 52 0.37 73, 000 6. 58 0. 23 74, 000 6. 570.37 80, 000 6. 59 0.14 75,000 It 6. 57 0. 09 ,400 1' 6. 87 0.76 80, 000j 6. 63 0. 51 84,

The exo gas used in the tests headed Test Piece Sintered in Exo Gas hada composition as follows:

The endo gas had the same composition as that given hereinabove.

in the examples summarized in Table I, run No. a employed nickel in theform of NiO having a particle size such that about 75% thereof was 325mesh. In run No. b, a different type of nickel was employed, also asNiO, but having a particle size such that about 90% was 325 mesh.

In run No. g 2% nickel was used in the form of Ni G which was very fine(pigment grade), so that all of it would go through a 325 mesh screen,the particles averaging about 6 microns in size. Some samples of thisrun, after the initial pressing operation, were heated at a temperaturein the range of about 1275 to about 1300 F. for 30 minutes, thenre-pressed at 100,000 psi. and sintered in the usual way as aforesaid togive a test piece having a density (as sintered) of 7.27 gms./cc. and atensile strength of 93,400 p.s.i. Some samples of the same run werefurther heat-treated at 1625 F. for about 20 minutes, then quenched inoil heated to F, then the temper drawn at 450 F. for about 30 minutes togive a final heat treated article having a density of 7.29 gms./ cc. anda tensile strength of 200,000 p.s.i.

Run No. h was one in which the nickel was present to the extent of about2% on the basis aforesaid, the nickel being in the form of nickel oxide(NiO) of a type in which about 75% was 325 mesh in particle size. Thisexample also included about /2 molybdenum and about 0.3% manganese. Somesamples of this run were also annealed and re-pressed as set forth forrun No. g to give an article having a density as sintered of 7.32gms./cc. and a tensile strength on an as sintered basis of 76,800 p.s.i.Some of these samples were further heat treated as set forth inconnection with run No. g to give final heat-treated articles having atensile strength of 190,000 p.s.i.

in run No. i the nickel (2% on a metal basis) was introduced in the formof nickel chloride (Nicl Following the initial pressing thereof, certainsamples were treated as set forth for run No. g above, includingannealing 30 minutes and re-pressing as stated to give an article havingon an as sintered basis a density of 7.13 gins/cc. and a tensilestrength of 90,600 p.s.i. Some samples of this same run were furtherheat-treated asset forth in detail as to run No. g to give finalarticles having tensile strengths of 183,000 p.s.i.

In run No. j the nickel used was the same as in run No. habove-described. Certain samples or this test run were re-pressed as setforth in run No. g above to given test pieces having a density of 7.30gms./cc. and a tensile strength of 79,200 p.s.i. on an as sinteredbasis. Some samples of this run were additionally heat-treated as set'1957, and hereinabove referred to.

oneness a1 forth in run No. g to give test pieces having tensilestrengths ofl85,000 p.s.i.

EXAMPLE II This example is given to illustrate the manner in whichpowder made in accordance with the present invention and particularly inaccordance with the preferred embodiment thereof given in Example Ithereof may be further treated to give it a very high tensile strength.

The first portion of this treatment was a pressing, annealing andre-pressing operation, wherein the first pressing was substantially thesame as given in Example I above, the blank being pressed at 60,000p.s.i. The annealing of this pressed blank was then carried on byheating the blank for about 30 minutes in a hydrogen atmosphere to atemperature in'the range of about l275 to 1300 F. The re-pressing of theblank so annealed was then effected by first cooling the blank, thenreinserting it in the same mold in which it was originally pressed andre-pressing it, this time at 100,000 p.s.i. The density of the pressed,sintered and re-pressed blank formed in this way was 7.3 gms./ cc.Several of these blanks were then sintered for a time and temperature asset forth in Example I above, so that this formed the first stage of theprocess. Some of the blanks formed in this way were tested and found tohave tensile strengths in the range of 75,000 to 100,000 p.s.i. andelongations of about 6% to 8%. 7

Others of the blanks formed in this way were then heated to 1625 F. for20 minutes in a non-oxidizing atmosphere, then quenched in oil, whichhad been preheated to a temperature in the range of 120 to 140 F, thentempered at various temperatures from 350 to 500 F., the tempering timebeing from 30 minutes to one hour (but being found not to be critical inany event). The heat treated and tempered blanks thus prepared weretested for tensile strength and found to have tensile strengths betweenabout 180,000 to 200,000 p.s.i.

EXAMPLE III This example is given to illustrate the differences betweenthe material produced in accordance with the present invention and thatproduced in accordance with applicants prior Eatent No. 2,799,570,granted July 16, Basically, these two processes differ in that thematerial in accordance with the prior patent and particularly includingthe iron ingredient thereof is co-reduced according to this prior patentalong with most of the alloying ingredients; whereas in the present casethe iron is introduced in a reduced or metallic state, but the presentcase is limited 'to a particular type of iron, namely, additive iron.

stant so that a comparison may fairly be made there between.

With the material of the present invention, the greatest shrinkage isfound to exist with blanks having the lowest values of the range ofgreen densities found in various samples (due to low compactingpressures) and was about 0.2%. As the green density of the blanks wasraised (by increasing the compacting pressure), the shrinkage wasreduced to a value of about 0.1%. With the material of the prior patentaforesaid, and with the lowest values of green density, the shrinkagewas slightly more than 0.6%; while with the highest values of greendensity, the shrinkage was still about 0.45%. It will be seen that withthe arbitrarily imposed limit of shrinkage which some users of powderedmetal have decided upon of 0.4%, the material of the present inventionwill 1500 F. under reducing conditions, so as to reduce all qualify;while the material of the prior patent Will not, 7

fact been found to be satisfactory from other points of view. 4

EXAMPLE IV This example is given to illustrate the comparative resultsin tests on the powder of the present invention as contrasted withpowders made up in exactly the same Way, but with the sole exceptionthat the iron powder used was not additive iron in accordance with thepresent invention, but rather was some commercially available type ofiron powder which, from the point of view of the disclosure of thiscase, may be termed subtractive iron. In each instance, the amount andcharacter of the alloying ingredients is exactly the same and includesabout 2% nickel and [2% each of molybdenum and manganese. In eachinstance the pressing was effected at 80,000 p.s.i. and the sinteringunder the same conditions as particularly set forth as in Example Ihere-of. The test'piece produced from the powder of the presentinvention had a tensile strength of 81,000 p.s.i.; whereas the testpieces produced using three different prior art iron powders availablecommercially were respectively 54,000, 60,000 and 62,000 p.s.i. One ofthese types of commercially available powder is made from iron ore andis generally termed Swedish sponge. Another is made by reducing millscale, but wherein the reduction is etfected under conditions such thatthe iron powder is properly described as subtractive and-is relativelyhard as contrasted with a relatively soft powder of the presentinvention.

EXAMPLE V This example is presented to illustrate the efiects of thevariation in the amount of nickel and also variation resulting fromdifferent manners of introducing the nickel in' the case. As such itgenerally supports the conclusions hereinabove set out as to the resultsof such variations. In the tests which are summarized in Table II whichfollows, various powderswere made substantially as set forth in ExampleI, but with the following particular variations. In each instance setout in Table II, the nickel was added as fine nickel oxide (NiO) with aparticle size such that about would pass through a 325 mesh screen. Ineach instance there was present in the powder about /2% molybdenum andabout 0.3% manganese. After mixing the ingredient materialstheywere'heated together at 1500 F. under reducing conditions, so as toreduce all the reducible materials present. The powders were tested asaforesaid, except that the pressing (to form test pieces) was done at80,000 p.s.i. and the sintering was done in an endo gas atmospherehaving the composition set out in Example I.

Table 11 Tensile Strength as Sintcred S rinkage,

EXAMPLE VI This example is presented to illustrate the effect of variousamounts of molybdenum on the properties of the powder. In all the testresults given in this example and specifically in Table III thereof.which follows, the powders contained 2% nickel, added'as nickel oxide,and 0.3% manganese added as manganese oxide. After mixing the ingredientmaterials, they were heated together at the reducible materials present.Since the purpose of molybdenum is to improve the heat treatabilitycharacteristic of the powder, some of the sintered bars were also 7 13heat-treated by beating them at 1850 F. for about 1 /2 hours, thenquenching them in oil, and drawing the temper at 650 F. for about 1 /2hours. The results of this series of tests are set out in Table HI.

14: different temperatures, the tensile strength of articles made atthose temperatures, and the respective shrinkages.

Table V Tensile Temperature, F. Strength Shrinkage,

(p.s.i) as percent sintered It is noted from the results of the testsset out in this example that the addition of molybdenum tends todecrease the amount of shrinkage and to increase the tensile strength ofthe articles as heat treated. It seems to have little or no effect uponthe tensile strength of the parts upon an as sintered basis.

EXAMPLE VII This example is presented to illustrate the effect ofvarious percentages of manganese in the powder. Except as to thisvariable, all the powders made and reported in this example included 2%nickel and /2% molybdenum added respectively as nickel oxide andmolybdenum oxide.

From the above it will be noted that there is relatively little changein tensile strength of test pieces with change in the reducingtemperature for the powder; but the shrinkage seems progressively todecrease with increasing reduction temperatures.

EXAMPLE IX This example is given to illustrate the effect ofprogressively increasing amounts of methane in the treating gas duringthe reduction of the reducible materials of the powder, it being notedthat in each instance the gas used is predominantly hydrogen. Theresults of a series of tests with different percentages of methaneadmixed with the hydrogen are given in Table VI.

Table VI The several samples made as set forth in this example 30 lTensie were reduced and parts were made therefrom by pressoHhpemntStrength Shrinkagg ing at 80,000 p.s.i. and sinterrng as 1n previousexamples. (p.s.i.) as percent The results of these tests are set out inTable IV which smiled follows: 3F 71 000 0 02 O Table IV 1 71,030 0. 200, 000 0. v 3 68,0)0 0.36 Tensile 4 71, 000 0. 46 Percent ManganeseStrength Shrinkage, 5 69, 000 0, 51

(p.s.i.) as percent sintered It is noted from the above that while thetensile 8 Zg ggg 8g strength of the finished parts varies withinrelatively 5: 77,000 1 narrow limits and without any definite trend, theshrink- 77,000 (3-63 age is progressively greater with increasingamounts of In each of the tests set out in this example, the manganesewas added in the form of electrolytic manganese powder.

A further test was made in which manganese was added as ferro manganesepowder with a total of 0.5% man ganese present in the finished powder.In this test, the resulting test piece was found to have a tensilestrength on as sintered basis of 80,000 p.s.i. and a shrinkage of 0.55%.

When the same amount of manganese was added in a further test in theform of manganese carbonate, other factors being the same, the tensilestrength of the test piece formed from the resulting powder was 83,000p.s.i. and the shrinkage 0.5%.

It may be concluded, therefore, that the variation in the amount ofmanganese has little effect upon the tensile strength on an as sinteredbasis; but that increased amounts of manganese tend to cause somewhathigher shrinkages. The manganese ingredient is added to impart desiredcharacteristics of heat treatability as hereabove set out, althoughthese factors per se are not illustrated in this example. In general, itis found, however, that the presence of manganese aids in permitting theparts to be hardened as set forth above.

EXAMPLE Vlil The purpose of this example is to illustrate the effects ofdifferent temperatures used during the reduction of the reduciblematerials or ingredients of the powder, the results being set forth inTable V showing the effect of such methane in the treating gases. Forthis reason, therefore, it is usually preferred that the proportion ofmethane be relatively low, on the basis that low shrinkage is usuallyone of the characteristics desired in the finished powder.

While there is herein specifically given data on but a few of the manytests which have been made with respect to this invention, and some ofthe alternatives have been pointed out or generally suggested, otheralternatives will occur to those skilled in the art from the foregoingparticular disclosure. We do not wish to be limited, therefore, exceptby the scope of the appended claims, which are to be construed validlyas broadly as the state of the art permits.

What is claimed is:

1. The process of preparing a powdered metal product for use in powdermetallurgy from a solid starting material consisting essentially of (1)iron, predominantly in the form of additive iron, and containing notover about 1% of impurities selected from the group consisting of theoxides of aluminum, silicon, titanium, chromium, calcium and magnesium,(2) metallic manganese, which is present in the amount of about 0.1% toabout 1%, (3) molybdenum in the form of at least one hydrogenreduciblecompound thereof selected from the group consisting of the oxides andchlorides of molybdenum, and which is present in the amount of about0.1% to about 1%, and (4) nickel in a form selected from the groupconsisting of (a) metallic nickel and (b) the hydrogenreduciblecompounds thereof which are selected from the group consisting of theoxides and chlorides of nickel,

and which is present in the amount of about /2'% to about 2 /2'%, allpercentages given being based upon the weight of the final product andall except that of the iron impurities being based on the respectivemetals as such; said process comprising the steps of blending togetherthe named materials in solid, fine-particle form to provide a startingmixture, reducing the hydrogen-reducible ingredients of said startingmixture by contacting said starting mixture While in a reducing zonewith a nonoxidizing gas containing a substantial proportion of hydrogenand while maintaining the temperature of all the materials in saidreducing zone in the range of about 1200 F. to about 1600 F. for a timesufficient to effect the reduction of substantially all thehydrogen-reducible material present; and cooling the remaining solidmaterial, following the reduction aforesaid, under non-oxidizingconditions and to a-temperature such that it may be exposed to theatmosphere without substantial spontaneous oxdiation, the powdered metalthus formed being capable of formation byconventional powder metallurgypractices into articles having high tensile strength and low shrinkagecharacteristics.

2. The process in accordance with claim 1, in which the iron ingredientof said solid starting material and at least a part of the manganese ofsaid solid starting material are derived from mill scale, which wasreduced by hydrogen in the presence of gaseous HCl to form said additiveiron.

3. The process in accordance with claim '1, in which the manganesecontent of said solid starting material (calculated as metallicmanganese as a percentage based on the total of the metals present) isabout /z%.

4. The process in accordance with claim 1, in which the molybdenumcontent of said solid starting material (calculated as metallicmolybdenum as a percentage based on the total of the metals present) isabout 72%.

5. The process in accordance'with claim 1, in which the nickel contentof said solid starting material (calculated as metallic nickel asapercentage' based on the total of the metals present) is about 2%.

6. The process in accordance with claim '1, in which said solid startingmaterial contains about /2% manganese, about /2% molybdenum and about 2%nickel, the percent'of each metal being a weight percentage based uponthe total weight of all the metals: present.

7. The process in accordance with claim 1, in which there is added tothe powdered metal produced as aforesaid about 0.6% to about 1% carbon,and then pressing the metallic powder mixture thus prepared to a desired7) form.

8. The process in accordance with claim 1, in which the temperature atwhich said materials are maintained in said reducing zones is about 1500F. and'the time in which said materials remain in the reducing zone-atthis temperature is about to minutes. v

9. The process in accordance with claim 7, comprising the further stepsof heating the pressed powdered metal product produced as aforesaid toatemperature in they powder metallurgy, comprising the steps ofpreparing a powdered metal product for, use therein from a solidstarting material consisting essentially of ,(1) iron, pre dominantly inthe form of additive iron, and containing not over about 1% ofimpurities selected from the group consisting of the oxides of aluminum,silicon, titanium, chromium, calcium and magnesium, (2) metallicmanganese, which is present in the amount of about 0.1% to about 1%, (3)molybdenum in the form of at least one hydrogen-reducible compoundthereof selected from the group consisting of the oxides and chloridesor" molybdenum, and which is present in the amount of about 0.1% toabout 1%, and (4) nickel in a form selected from (a) metallic nickel and(b) the hydrogen-reducible compounds thereof which are selectedrfrom thegroup consisting of the oxides and chlorides or nickel, and which ispresent in the amount of about to about 2 /2 all percentages given beingbased uponthe weight oi the final product and all except that of theiron impurities being based on'the respective metals as such; saidprocess comprising the steps of blending together the named materials insolid, fine-particle form to provide a starting mixture, reducing thehydrogen-reducible ingredients of said starting'rnixture by contactingsaid starting mixture while in a reducingzone with a nonoxidizing gascontaining a substantial proportion 'of hydrogen and While maintainingthe temperature of all the materials in said reducing zone in the rangeof about 1200" F. to about 1600 F. for a time su-fiicient to effect thereduction of substantially all the hydrogen reducible material present;cooling the remaining solidmaterial, following the reduction aforesaid,under nonoxidizing conditions andto atemperature such that it may beexposed to the atmosphere without substantial spontaneous oxidation;forming the powder to a desiredshape forthe article to be made bypressingrit in a mold of a shape to form the desired article and at apressure in the range of about 50,000 to about 100,000 p.s.i., annealingthe pressed article at a temperature in the range of about 1275" toabout 1800 F. in a nonas aforesaid to a temperature in the range ofabout 1500 to about 1600 F. for about 15 to 20 minutes, quenching thethus-heated articlein a quenching oil which has been heated prior to thequenching to the temperature range of about to about F, jandtemperingthe quenched article by holding it at a selected temperature in therangeof about 400 to about 600 F.

References Cited in the file of this patent UNITED STATES PATENTS1,453,057 Williams Apr. 24, 1923 2,295,334 Clark Sept. 8, 1942 2,352,316Goetzel June 27, 1944 2,744,002 Crowley May 1, 1956 2,799,570 Reed etal. July 16, 1957 2,811,433

Whitehouse et al. Oct. 29, 1857

1. THE PROCESS OF PREPARING A POWDERED METAL PRODUCT FOR USE POWDERMETALLURGY FROM A SOLID STARTING MATERIAL CONSISTING ESSENTIALLY OF (1)IRON, PREDOMINATLY IN THE FORM OF ADDITIVE IRON, AND CONTINING NOT OVERABOUT 1% OF IMPURITIES SELECTED FROM THE GROUP CONSISTING OF THE OXIDESOF ALUMINUM, SILICON, TITANIUM, CHROMIUM CALCIUM AND MAGNESIUM, (2)METALLIC MANGANESE, WHICH IS PRESENT IN THE AMOUNT OF ABOUT 0.1% TOABOUT 1%, (3) MOLYBDENUM IN THE FORM OF AT LEAST ONE HYDROGENREDUCIBLECOMPOUND THEREOF SELECTED FROM THE GROUP CONSISTING OF THE OXIDES ANDCHLORIDES OD MOLYBDENUM, AND WHICH IS PRESENT IN THE AMOUNT OF ABOUT0.1% TO ABOUT 1%, AND (4) NICKEL IN A FORM SELECTED FROM THE GROUPCONSISTING OF (A) METALLIC MICKEL AND (B) THE HYDROGENREDUCIBLECOMPOUNDS THEREOF WHICH ARE SELECTED FROM THE GROUP CONSISTING OF THEOXIDES AND CHLORIDES OF NICKEL, AND WHICH IS PRESENT IN THE AMOUNT OFABOUT 1/2% TO ABOUT 21/2%, ALL PERCENTAGES GIVEN BEING BASED UPON THEWEIGHT OF THE FINAL PRODUCT AND ALL EXCEPT THAT OF THE IRON IMPURITIESBEING BASED ON THE RESPECTIVE METALS AS SUCH; SAID PROCESS COMPRISINGTHE STEPS OF BLENDING TOGETHER THE NAMED MATERIALS IN SOLID,FINE-PARTICLE FORM TO PROVIDE A STARTING MIXTURE, REDUCING THEHYDROGEN-REDUCIBLE INGREDIENTS OF SAID STARTING MIXTURE BY CONTACTINGSAID STARTING MIXTURE WHILE IN A REDUCING ZONE WITH A NONOXIDIZING GASCONTAINING A SUBSTANTIAL PROPORTION OF HYDROGEN AND WHILE MAINTAININGTHE TEMPERATURE OF ALL THE MATERIALS IN SAID REDUCING ZONE IN THE RANGEOF ABOUT 1200*F. TO ABOUT 1600*F. FOR A TIME SUFFICIENT TO EFFECT THEREDUCTION OF SUBSTANTIALLY ALL THE HYDROGEN-REDUCIBLE MATERIAL PRESENT;AND COOLING THE REMAINING SOLID MATERIAL, FOLLOWING THE REDUCTIONAFORESAID, UNDER NON-OXIDIZING CONDITIONS AND TO A TEMPERATURE SUCH THATIT MAY BE EXPOSED TO THE ATMOSPHERE WITHOUT SUBSTANTIAL SPONTANEOUSOXIDIATION, THE POWDERED METAL THUS FORMED BEING CAPABLE OF FORMATION BYCONVENTIONAL POWDER METALLURGY PRACTICES INTO ARTICLES HAVING HIGHTENSILE STRENGTH AND LOW SHRINKAGE CHARACTERISTICS.